This invention relates to a new and useful process for preparing aromatic aldehydes by the direct introduction of the formyl group into a benzene ring system. More particularly, it is concerned with a novel two-step method for synthesizing a 5-substituted-2-methoxybenzaldehyde compound wherein the substituent group is either isopropyl or trifluoromethoxy. These two aldehyde compounds are known to be of value as intermediates that lead to certain substance P receptor antagonists.
In accordance with the prior art, there has already been described certain compounds which are known to be of value as substance P receptor antagonists. Included among these are such nitrogen-containing heterocyclic ring compounds as (2S,3S)-cis-3-(2-methoxy-5-trifluoromethoxybenzyl)amino-2-phenylpiperidine , which is described and claimed by J. A. Lowe, III et al., in Published P.C.T. International Patent Application No. WO 93/00331 (published Jan. 7, 1993) and (2S,3S)-cis-2-(diphenylmethyl)-N-[(2-methoxy-5-isopropylphenyl)methyl]-1-a zabicyclo[2.2.2]octane-3-amine, which is described and claimed by F. Ito et al., in Published P.C.T. International Patent Application No. WO 92/21677 (published Dec. 10, 1992). Both compounds are useful as non-steroidal anti-inflammatory (N.S.A.I.) agents, being of specific value in the treatment of arthritis, asthma and inflammatory bowel disease.
In the past, these particular compounds have been prepared by various synthetic means but essentially by a method which involves the reductive amination of the appropriate aldehyde compound, i.e., by reacting either 2-methoxy-5-isopropylbenzaldehyde or 2-methoxy-5-trifluoromethoxybenzaldehyde, as the case may be, with the corresponding heterocyclic 3-amino compound in the presence of a source of hydrogen, or else by first condensing the aforesaid 3-amino compound with the aldehyde and then reducing the resulting imine intermediate to ultimately give the key benzylamine side chain. The starting aromatic aldehyde component in this particular reaction scheme had always been prepared in two steps starting from the corresponding known and readily available 4-substituted phenol compound and this, in turn, initially involved (1) first methylating the phenol compound with methyl iodide in an acetone solvent medium in the presence of solid potassium carbonate, followed by (2) direct formylation of the resulting 4-substituted methylated phenol (i.e., 4-substituted anisole compound) with .alpha.,.alpha.-dichloromethyl methyl ether in a methylene chloride solvent system in the presence of titanium tetrachloride as catalyst. However, this particular two-step method for the production of the aldehyde suffers from the drawback of being conducted in a non-homogenous reaction system in the first step, with all its attendant disadvantages, and in employing the somewhat hazardous titanium tetrachloride reagent as catalyst in the second step. In the latter connection, it should be noted that certain stringent safety requirements are normally called for when handling the latter agent, particularly when unit operations are conducted on a large scale. Additionally, the use of various hazardous waste disposal techniques are also required for the removal of the titanium tetrachloride byproducts that are usually formed in the aforesaid aromatic formylation reaction.
In the past, F. Merger et al. in U.S. Pat. No. 4,192,949 indicate that they have prepared various methyl phenyl ethers, including both 4-methylanisole and 4-methoxyanisole, from the corresponding phenol compounds, using dimethyl carbonate in the presence of a tertiary-amine base as catalyst without the presence of a solvent. Although the Merger et al. patent also includes p-isopropylphenol in a long list of many other possible phenolic starting materials for the aforementioned reaction, there is no indication that 4-isopropylanisole was ever actually prepared in this particular manner. On the other hand, W. E. Smith in the Journal of Organic Chemistry, Vol. 37, No. 24, p. 3972 (1972) reports on the direct C-formylation of several aromatic compounds, including 2,6-dimethylanisole, via a method which involves the use of hexamethylenetetramine in trifluoroacetic acid in a modified Duff reaction, but there is no indication in the aforesaid paper by Smith that such a reaction could ever be successfully carried out using other non-acidic derivatives of anisole as substrate. In particular, there is no indication that the reaction of Smith could be applied to parasubstituted derivatives of anisole.